1. Field of the Invention
This invention generally relates to a musical tone generating apparatus for use, for example, in an electronic musical instrument, and more particularly to such a tone generating apparatus in which a data compression method based on the linear predictive coding is used to reduce the amount of data of tone signals.
2. Prior Art
There have been proposed various kinds of signal processing methods in which an analog signal is first converted into sampled digital data to allow the analog signal to be digitally processed. Such methods are widely used particularly in tone generating apparatuses provided in, for example, voice synthesizers, electronic musical instruments and data communication systems.
Among such signal processing methods, a PCM (Pulse Code Modulation) method such as one disclosed in U.S. Pat. No. 4,383,462 is known. In this PCM method, an overall waveform of an analog signal such as a tone signal is sampled at a predetermined rate and the thus obtained series of sampled digital data are stored in a memory. And, the stored data are sequentially read from the memory and converted into analog signals to reproduce the analog signal. The PCM method is advantageous in that an exact reproduction of signal can be achieved and is therefore suitable for use in an electronic musical instrument, but has such a deficiency that the amount of data to be stored in the memory is large.
To overcome the deficiency of the PCM method, various methods of reducing the amount of data, such as a DM (Delta Modulation) method, a DPCM (Differential Pulse Code Modulation) method and an ADPCM (Adaptive Differential Pulse Code Modulation) method, have been proposed. Even by these methods, however, the amount of data can not sufficiently be reduced to such an extent that an electronic musical instrument can employ these methods to generate various musical tone signals. Also, in these methods, each data to be presently stored is formed based only on the precedingly obtained sample data and therefore includes unavoidable errors so that an exact reproduction of signal can not be achieved.